This invention pertains in general to a system for detecting the presence of failed fuel elements within the core of a nuclear reactor and more particularly, to an on-line system for diagnosing the identity and condition of breached fuel elements in a nuclear reactor.
The reactor fuel in a fission type reactor is typically an isotope of uranium, such as uranium 235. The reactor fuel may take the form of a fluid, such as an aqueous solution of enriched uranium; but typically the fuel is solid, either metallic uranium or a ceramic such as uranium oxide or uranium-plutonium oxide. The solid fuel material is fabricated into various small plates, pellets, pins, etc; which are usually clustered together in an assemblage called a fuel element. Almost all solid fuel elements are clad with a protective coating or sheath that prevents direct contact between the fuel material and the reactor coolant. The cladding also serves as part of the structure of the fuel elements.
The operation of the fuel elements generates heat, which heat is typically dissipated by means of a coolant passed through the reactor. The coolant can be water operating as either liquid or steam, or the coolant can be a liquid metal such as sodium or a sodium-potassium mixture. The coolant passes in proximate contact over the cladded-fuel elements; and sound cladding isolates or separates the coolant from the radioactive fuel material. However, in the event of a breach in the cladding, the coolant directly contacts the fuel. The radioactive discharge may then in turn be conveyed via the coolant throughout the entire coolant system thereby contaminating the entire system.
Also given off, as part of the radioactive discharge, are at least nine different isotopes that not only give off typical gamma rays of radioactivity, but also give off what are known as delayed neutrons. These isotopes, or delayed neutron emitters, would include bromine, iodine, and tellurium to name a few. Each of these delayed neutron emitters is soluble in liquid sodium (the coolant) so that it readily blends in with the coolant, should a fuel element cladding breach occur, and flows from the coolant throughout the system.
Therefore, it becomes readily apparent that the event of fuel cladding breaches must be taken into account when designing and operating a nuclear reactor. A quick and precise diagnosis of cladding breach events would ensure that the reactor operator would correctly respond upon the occurence of such a breach. A precise diagnosis of the condition of a breached pin would introduce significant advantages for reactor plant operation. The reactor could be safely operated under such breached pin conditions until a predetermined allowable radioactivity limit is exceeded. In the event that this predetermined limit is not exceeded, the reactor could be safely operated until the next scheduled discharge. Further, if a system could accurately diagnose whether a cladding breach is stable or unstable, the reactor operator could continue the operation of the reactor in the event of a stable breach and shut down the reactor upon the occurrence of an unstable breach. The continued operation of the reactor under a stable breached pin condition could significantly improve reactor availability.
Conventionally, radioactive elements which have mixed with the coolant are detected by means of a GeLi detector (a germanium and lithium gamma-ray detector) incorporated into a GLASS (a germanium-lithium argon scanning system), or other readily available detecting systems. These systems are used to detect the activity in the cover gas of the reactor. Typically however, these systems have been used only as an annunciation of fuel failure. After identification of a "gas leaker" (a breached fuel element), fission gas activity is typically removed by a plant cleaning system, such as the cover gas cleanup system (CGCS) used in EBR-II. This system removes fission gas activity from the reactor cover gas by semicontinuously extracting part of the cover-gas, cleaning it cryogenically and returning it to the core. The CGCS allows the reactor to be operated after a breach has occurred in a fuel element. However, the system effectively obscures any information about the failure which may be contained in the gas release data obtained from a GLASS.
Therefore, typically used in conjunction with CGCS are delayed neutron systems such as the system disclosed in U.S. Pat. No. 4,415,524 issued to K. C. Gross et al., and/or fuel element failure location systems, such as the gas tagging system disclosed in U.S. Pat. No. 4,495,143, issued to K. C. Gross et al., which monitor the identity and condition of breached pins. However, such systems provide only very qualitative information about the type of breached fuel involved (oxide or metal), and its burnup (high or low).
The run beyond clad breach mode operation of a commercial liquid metal reactor may not be allowed without an on-line identification of breached pins and a diagnosis of the breached pin condition and development. The diagnosis of a breached pin should include a reliable prediction of the on-going condition of the event.
Therefore, in view of the above, it is an object of the present invention to provide an on-line apparatus and method for diagnosing the severity of a breached fuel element.
It is another object of the present invention to provide an on-line apparatus and method for determining the number of breached fuel elements.
It is another object of the present invention to provide an apparatus and method for determining the mode of gas released from a breached fuel element.
It is a further object of the present invention to provide an apparatus and method for determining the breaching mechanism in a fuel element.
It is still a further object of the present invention to provide an apparatus and method for determining if a breach in the cladding of a fuel element is benign or unstable.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.